1. Field of the Invention
The present invention relates to color wheels used for projectors to generate color sequential illumination. The present invention relates as well to projectors using such color wheels.
2. Description of Related Art
Devices of the aforementioned type are used in applications where periodic color changes need to be produced in rapid sequence. Examples of optical systems using such devices are picture generation devices or display arrangements. Rear or front projection systems for television systems are typical applications.
In order to produce the required rapid color changes, color filters are inserted into the optical light path of a light beam in rapid sequence. For this purpose a carrier is used that is equipped with circularly arranged filter segments on the periphery. This arrangement forms a color rotor with parts of the segments radially extending over the carrier and thereby forming a color ring to be inserted into the optical path. This color rotor is rotated around its central axis. Through the rotation of the color rotor the filter segments are alternatingly inserted into the optical path and removed from it. Thereby the desired rapid color change is produced. In order to realize this rotation the color rotor is fixed to a motor. Color rotor and motor form a color wheel.
Because the picture generation device must be able to produce a high picture quality, the color changes must be executed very rapidly. This means that the filter segments must be moved through the optical path of the light beam at very high speed. Fast rotation of the color rotor is therefore mandatory. As a result large forces due to accelerations which are several hundred times larger than the acceleration g due to gravity act on the color rotor and particularly on the sensitive filter segments; for particularly high picture qualities the accelerations can exceed 1000 g. Such picture generation devices must also satisfy very high brightness requirements which can only be achieved with powerful light sources. Through these powerful light sources the device is subjected to correspondingly high temperatures of up to 100° C.
In addition the radial concentricity of the device must be highly accurate in order to achieve a long operational life of the device. Therefore special care must be taken in balancing such color wheels.
However broad-scale utilization in so-called low-cost display applications is only feasible if it becomes possible to produce the color wheel at very low costs despite the high quality requirements. Therefore no complicated and expensive balancing methods may be applied.
There are different schemes of balancing methods known in the art. Static as well as dynamical balancing. Single and dual plane balancing.
In a classical static approach the rotor of the color wheel is rotated and the unbalance is measured. Unbalance is always present if the center of mass of the rotor does not coincide with the rotational axis. This can be corrected by either removing mass from the rotor at some point distant to the rotational axis (negative balancing) or by adding mass to the rotor at some point (positive balancing).
In order to perform negative balancing, typically holes are drilled into the carrier. If this is done while the color rotor remains connected to the motor there is a good chance that the bearings suffer from the forces they experience during the removal of the mass. One could as well think about removing the rotor form the motor before drilling the holes. Unfortunately this is quite time consuming and therefore renders the balancing method expensive. This is especially true if balancing needs to be performed in an iterative way (rotation-first correction-rotation-second correction . . . ).
In order to perform positive balancing additional balancing weight is attached to the rotor. For color wheels this is typically done by adhering a lump of adhesive to the carrier. If the weight of the adhesive is not enough, the adhesive is used to glue other material with a higher density, such as a piece of steel to the carrier. Performing the balancing procedure by such a step requires a high degree of accuracy and experience, since the adhesive tends to smear out, flowing to parts of the rotor where it should not go to. Therefore the carrier sometimes comprises a circular groove in order to receive the adhesive. This however limits the degree of freedom for positioning the balancing mass. In addition hardening of the adhesive takes in time before the rotor may be brought to rotation again. This makes the method quite expensive for example if balancing needs to be performed in an iterative way as described for the negative balancing.
According to a dynamical balancing method known in the art the carrier or another part of the rotor comprises a container with a ring shaped volume ready for receiving the balancing mass. Before rotating the rotor the adhesive and if necessary for example some metal beads are inserted into the container. The amount of adhesive is kept small enough in order not to fill completely the container. If such an assembly is rotated, the adhesive, together with the beads if present, will automatically flow to the correct axial position in order to minimize the unbalance. The adhesive needs then to be hardened. Care has to be taken, that the adhesive remains at the same position after rotating and before and/or during hardening the adhesive. Here as well this is one degree of freedom less for the position of the balancing mass since the radial position is predetermined by the container.
The balancing methods described until now exclusively refer to single plane balancing. This is often sufficient, especially if the rotor may be well approximated by a thin plate and if rotational speed is moderate. However for color rotors comprised in color wheels the rotational speed is often in the range of 7000 rpm to up to 15000 rpm. In this cases single plane balancing is sometimes no more sufficient and balancing has to be performed for at least two well separated planes perpendicular to the rotational axis.
Here as well the different schemes such as negative balancing, positive balancing or dynamical balancing may be applied, with all the advantages as well as disadvantages of the respective methods.
It is the objective of the present invention to provide for a color wheel which allows for a simplified balancing procedure. This method should at least partially overcome the disadvantages of the methods already known in the art.